KARSTIFICATION IN MIOCENE GYPSUM - AN EXAMPLE FROM SIVAS, TURKEY

In this study, karstification developed in the Miocene gypsum which co vers a large area around Sivas, Turkey, the relation between regional tectonics and karstification, and hydrogeological features have been i nvestigated. The karstic features in the gypsum have developed conform ing to the fault zones and the general strikes of gypsum beds. In the study area, numerous dolines (sinkholes) and ponors (swallow holes) of different sizes are observed. Most of these karstic features are in t he different-sized longitudinal depressions (troughs) which conform to the tectonic structures. These karstic features occur NE-SW along bed ding planes and about NW-SE and NE-SW along fault zones. At the inters ections, high-capacity (yield) karst springs (Goydun and Seyfe springs ) are observed. The Goydun and Seyfe springs issue from the karstified gypsum aquifer, with an average discharge of 1.10 m(3)/s and 0.25 m(3 )/s, respectively. In addition, there are some low-yield (a few l/s) s prings in the same drainage area. The surface drainage area of the spr ings is 64 km(2), and the precipitation in this area cannot provide th e total groundwater discharging from Goydun and Seyfe springs. Water b udget calculations indicate that more than 70% of the water discharged by these two springs is provided from the adjacent basins. The Goydun and Seyfe springs are rich in dissolved solids and average electrical conductivities (EC) are about 13 000 mu S/cm. The major cations in th e waters are Ca and Na; major anions are SO4 and Cl, and the waters ar e brackish. Because of these properties, especially in summer and autu mn, the springs cause extreme salinization in Kizilirmak river.